feat(notify): add percentage threshold type for balance low notification

- Add threshold_type field (fixed/percentage) to system and user settings
- Add total_recharged field to users table, auto-incremented on balance credit
- Percentage mode: effective threshold = total_recharged × percentage / 100
- User-level threshold_type inherits from system default when not set
- Update admin settings UI with radio selector (fixed amount / percentage)
- Migration: 102_add_balance_notify_threshold_type.sql

backend/ent/migrate/schema.go

@@ -1079,8 +1079,10 @@ var (
 		{Name: "totp_enabled", Type: field.TypeBool, Default: false},
 		{Name: "totp_enabled_at", Type: field.TypeTime, Nullable: true},
 		{Name: "balance_notify_enabled", Type: field.TypeBool, Default: true},
+		{Name: "balance_notify_threshold_type", Type: field.TypeString, Default: "fixed"},
 		{Name: "balance_notify_threshold", Type: field.TypeFloat64, Nullable: true, SchemaType: map[string]string{"postgres": "decimal(20,8)"}},
 		{Name: "balance_notify_extra_emails", Type: field.TypeString, Default: "[]", SchemaType: map[string]string{"postgres": "text"}},
+		{Name: "total_recharged", Type: field.TypeFloat64, Default: 0, SchemaType: map[string]string{"postgres": "decimal(20,8)"}},
 	}
 	// UsersTable holds the schema information for the "users" table.
 	UsersTable = &schema.Table{